As is known, in the course of making CMOS devices, subsequently to forming the gate and the source and drain regions of the device, and related re-oxidation, a dielectric layer of an isolating oxide is deposited to a thickness in the range of 0.8 to 1.0 micrometers.
That dielectric layer, commonly referred to as dielectric of first interconnection, separates the surface layer of polycrystalline silicon from the metallic paths of electrical interconnection.
Once the gate of the device has been defined, the device shows a predetermined map.
The successive deposition of the dielectric layer of first interconnection follows this map enhancing, however, its features affect the subsequent masking, contact forming, and metal deposition steps result in a low yield of current manufacturing processes due to high rate of faulty devices. In addition, surface irregularities on the mapping represent a major hindrance to a reduction in size of the interconnection paths to the metallization mask.
To obviate this drawback, the prior art has proposed, for CMOS devices, the use of a double dielectric layer formed of a first thickness of silicon dioxide over which a layer of borophosphosilicate is deposited.
Borophosphosilicate (i.e. borophosphosilicate glass or BPSG) allows the underlying map to be softened if the semiconductor is subjected to heat treatment at 900.degree.-1,000.degree. C. for about 30-60 minutes.
In accordance with a method known as classic planarization, for example as described in the "5th International IEEE V-Mic Conference" No. 357, June 13-14, 1988, the borophosphosilicate is coated with a photoresist layer and plasma attacked with selectivity 1a1. A fairly planar surface is thus obtained; however, the reproducibility of this process is poor and the process itself non-uniform, which is inconsistent with the requirements of of very large volume integrated circuit manufacturing processes. Also, the extent of the planarization to be achieved thereby is heavily dependent on the underlying mapping, and in addition, two extra steps have in all cases to be provided in the manufacturing process.
A second prior technique, as described for example in the "5th International IEEE V-Mic Conference", No. 293, June 13-14, 1988, consists of superimposing, on the borophosphosilicate, a layer of siloxane, or so-called spin-on-glass (SOG), which shows to be less sensitive to the underlying mapping during the selective attack step. However, here too does the manufacturing process involve extra steps.
A further prior approach is described in the "4th International IEEE V-Mic Conference", No. 61, June 15-16, 1987, and consists of depositing the siloxane directly onto the map surface, and hence on the layer of polycrystalline silicon, and of applying a heat treatment under a nitrogen medium followed by deposition of a borophosphosilicate layer. Further final heat treatment at 920.degree. C. is found to soften the profile contour of the end surface. However, not even this approach has proved quite effective to yield planarized structures, despite any advantages to be derived therefrom.